HID- class USB Serial Communication for AVRs using V- USB « RAYSHOBBY. NETA month with no new post? That’s unacceptable. Since my last trip to Shenzhen, I actually have quite a few new ideas and projects to post about. The thing is there are so many of them that I don’t know which one to start with! Anyways, I’ve decided to put a stop to this.

In this blog post, I will describe some work I did a little while back about implementing HID- class serial communication for AVR microcontrollers using the V- USB library. Introduction. First, let me explain what I am trying to do. As you probably know, V- USB is a very useful software- only implementation of low- speed USB device for AVR microcontrollers. It adds USB functionality for almost any AVR, particularly for those without hardware USB functionality. With this, it’s possible to make a very low- cost standalone Arduino with USB port and without having to use an FTDI chip. I know there is the Arduino Leonardo, which is based on ATmega. USB functionality.

But mega. 32u. 4 only exists in SMT package, and it’s more expensive than mega. Besides, I am fully embracing the ? Mostly for two reasons. One is to flash a program to the microcontroller, so it needs a USB- based bootloader. For this, there is a very nice open- source project called USnoobie, which can bootload mega.

USBasp programmer. This way you can flash a program through the USB port directly, without using a serial cable or an external AVRISP programmer. So this is all good. The second reason to have USB is for serial communication — the ability to transfer data (e. This is useful especially for debugging (i. There are some related projects.

For example, I came across the AVR- CDC project, which turns a mega. CDC- class USB- serial converter. But there seem to be some limitations of using V- USB to implement CDC (i. I would like to make a HID- class USB device which does not require driver installation. So overall I didn’t find any available resource that I can use directly. Circuit Design and V- USBNow I’ve explained the motivation, let’s see how to go about implementing it. The first step is to learn to use V- USB.

I started with the Easy. Logger project downloaded from the V- USB website. It is based on the tiny. The program reads a sensor (e. This is an excellent starting point for me because USB keyboard is a standard HID- class device, and the project is simple enough that I can easily learn and make modifications. To adapt it to mega. I first made a circuit based on USnoobie.

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Here is the schematic and my build of the circuit on a breadboard: It’s a pretty standard V- USB setup. I assigned digital pin PD2 (INT0) to USB D+, pin PD7 to USB D- , and PD4 to a pushbutton.

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The pushbutton is used to enter bootloading mode. Specifically, if the button is pressed when the circuit is powered up, the bootloader is activated and the mcu will appear as a USBasp programmer. Different from USnoobie, I’ve decoupled this button from the D- line, so that I can use the button for general- purpose input (otherwise pressing the button will trigger the D- line). This requires changing the USnoobie code slightly to use pin PD4 for bootloading condition. Finally, I’ve also added a MCP9.

ADC0 for testing later. The next step is to modify the source code. Download Dance Gopher Caddyshack Youtube Scenes. First, change usbconfig. D+ and D- pin settings, specifically the following three macro defines: #define USB. This step is pretty technical and tedious. It mainly involves changing register names to match mega. Also, the calibrate.

Oscillator(); function can be removed as the mega. MHz clock. I also modified the Makefile in order to compile and flash the code for mega.

After a few tweaks here and there, the Easy. Logger started working on my mega. It can successfully output numerical values to a text editor through the USB port. This is very encouraging.

If these steps don’t make much sense, you can take a look at the code below, and give it a try yourself. Learning HIDHID stands for Human Interface Device.

It’s a USB class designed primarily for keyboard, mice, joystick, and similar human interface devices. The nice thing about HID is that it’s supported on all operating systems. For example, on Windows, the system uses built- in HID driver to handle USB requests, so no driver installation is required. This is why when you plug in a keyboard or mice, you never have to install a driver (imagine how annoying it would be if you had to!). To implement HID, you first will need to construct a HID descriptor, which describes the number of reports, and the size, meaning, and (optionally) value range of each report. For example, these reports can be the ASCII code of the pressed key, the x and y offsets, and button presses of the mouse. There are also more general- purpose reports like a buffer of bytes.

This is what I will be using to transfer bytes in and out between the device and host. To be honest, the whole USB descriptor thing was very obscure to me in the beginning.

I didn’t know if there is one correct way to define it, or it can be flexible. As it turns out, Linux is pretty forgiving about it, but Windows is not. After many trials and errors, I finally settled with this HID descriptor: PROGMEM constcharusb. Hid. Report. Descriptor. As I said above, Linux is pretty flexible about the descriptor — you can change it in many ways and it still works.

Windows, however, is very strict — if you are not careful, it will simply refuse to recognize the device. The next step is to write functions to handle the USB requests. For transferring data out, I used the usb. Set. Interrupt function, which allows sending data spontaneously to the host (i. So everything gets compiled in the Arduino software.

You can certainly use avr- gcc to compile the code as well). I made HIDSerial an inherited class from Arduino’s Print class, so I can make use of the many available print functions (e. For transferring data in, I implemented the usb. Function. Write function, as described in the V- USB Document Wiki. Now, before I can test and debug the code, I need to have some minimal host software to communicate with the device.

That’s what I will be describing next. Write Host Software using Processing. Going the HID route means the device will not appear as a standard serial COM port, so you can’t use the standard serial monitor to send and receive values. Instead, I will have to write host software myself. I can certainly do this in C or Java.

But since I want to make the host software cross- platform, I have chosen to implement it in Processing, which allows me to easily export the program as standalone applications on all platforms. Long story short, to do this, I used HIDAPI library.

It has all the goodies to handle communications with HID devices, such as finding a device, opening the device, reading from the device, and sending feature report to the device. Also, using the G4. P library, you can easily build a GUI with buttons and text fields, and make the interface just like a standard serial monitor. Once the software is finalized, I can simply click on !

Below are two screenshots of the HID serial monitor: Source Code. The source code of this project is available for download on my Git. Hub repository: You can either do a git clone, or directly download the project as a zip file (see the Download . The folder contains circuit schematic, part list, Arduino library (HIDSerial), host software (HID serial monitor), and bootloader (optional, but recommended as it allows you to re- flash the microcontroller through USB, without any external programmer). The Arduino library consists of several starting examples, which are also demonstrated in the video above.

I am sure there are bugs and issues with the code, but please feel free to leave comments and feedback below, in order for me to improve the code. Limitations and Issues.

The biggest limitation of this implementation is the data transfer speed — it’s much slower compared to a standard USB- serial converter. This will be an issue if you need to pump out data as fast as possible.